Band-pass filter circuits



Jan. 12, 1943. R. B. ARMSTRONG ETTAL 2,398,258

BAND-PASS FILTER CIRCUITS Filed Sept. 28, 1940 DEZ/AMR V lNVENTOR-S Rig-ARMSTRONG G-L. GRISDALE BY 5. F.)GO DENOUGH I s ATTORNEY an inductance L2 and a Patented Jan. 12, 1943 BAND-PASS FILTER CIRCUITS Reginald Basil Armstrong, Chelmsford, George Lambert Grisdale, Great Baddow, and Ernest Frederick Goodenough, Springfield, Chelmsi'ord, England, assignors to Radio Corporation oi America, a corporation of Delaware Application September 28, 1940, Serial No. 358,778

In Great Britain October 5, 1939 5 Claims. 179171) Ihis invention relates to band-pass filter circuits and particularly to variable selectivityfilter circuits employing crystal resonators coupling two tuned circuits. v

According to the invention a band-pass .filter comprises two tuned "circuits, crystal resonator means coupling said tuned circuits, means for neutralising the capacity of said crystal resonator, means for short circuiting said crystal resonator, and switch means operable to control the effective impedance of said tuned circuits in such manner that in one position of said switch means said tuned circuits are tuned to the series-resonant frequency of said crystal and in another position of said switch means said tuned circuit are tuned one above and the other below said series-resonant frequency.

The means for short circuiting the crystal resonator and the switch meansmay be so interrelated that the said switch is effective to control the impedance of said tuned circuits when said crystal resonator is not short circuited.

Where the band pass filter is applied to the first inter-valve stage or a multi-stage intermediatefrequency amplifier switch means may be provided such that when the crystal resonator is included-in the said band-pass filter the value of the coupling in subsequent stages is constant and that when said crystal resonator is shortcircuited the value of the coupling is variable wherebythe overall selectivity-response is varied.

The invention is illustrated in the accompany-- ing drawing whereof Figure 1 is a circuit diagram of an arrangement invention, and Figure 2 is a circuit diagram of a second embodiment.

In the circuit carrying the invention into effect as applied to the first intermediate frequency amplifier, and as illustrated in Figure 1, the anode of a valve VI is connected as is usual to a tuned circuit comprising an inductance LI and condenser Cl allel, the remote end of the inductance being connected to a source of anode potential. The anode is also capacity coupled by condenser C4 to one electrode of a crystal resonator Q,- series resonant at the desired intermediate frequency.

The other electrode of the crystal resonator is connectedto a tuned input circuit, comprising capacity 02 of a second valve V2grid bias being applied by way of the inductance of said input circuit. The said other electrode of the-crystal is also connected to the capacity element of a neutralisingcircult comprising a capacity 03 and inductance L2 in series,

in accordance'with the in par...

. the inductive element being coupled to the in- S2. The blade of the change over switch S2 is connected to one terminal of a condenser C5 the other terminal of which is conected to earth.

In one position of the change-over switch S2,

and when the short-circuiting switch SI is open, this last mentioned condenser C5 is eifectively "in parallel with the condenser CI .of the first intervalve stage of an and capacity oisaid transferrable condenser 05 tuned circuit and the two tuned circuits Ll Cl, L2 02 are then tuned to the intermediate frequency, and in the other position of the changeover switch S2 the last mentioned condenser C5 is removed from the first tuned circuit and connected effectively to parallel with the conden'ser C2 of the second tuned circuit. In this last condition the first and second tuned circuits ,Ll Cl, L2 C2 are tuned respectively above and below the intermediate frequency. to a like amount, depending inter alia upon the value of the condenser 05 transferred. When the crystal is short circuited, this condenser is in parallel with those (CI, C2) in both tuned circuits.

Inanother embodiment of the invention illustrated in Figure 2, as applied to a multi-stage intermediate frequency amplifier, the change over switch Sl2 is provided with a plurality of contacts whereby, in one position (position 2) of the switch, theicrystal Q is in circuit and the is eflectively in parallel with the capacity CI of -the first tuned circuit, in another position (position 1) the crystal is in circuit and-the transferrable condenser 05 is eiiectively in parallel with the capacity C2 of the second tuned circuit, and ma third position (3) the crystal is short-circuited, and in all these positions the coupling insubsequent intervalve stages remains constant. In subsequent positions. of the change over switch Sl2 the cou-' pling of the subsequent stageor stages is varied. This may be effected by providing' the change over switch with a plurality of sets of arms Sl2'-,

SH" and sets of contacts (or gang controlling a number ofiurther switches with the changeover switch). The further sets of contacts are connected to tapping points on the inductance L2),- L2)! stages and the switch 'arm is connected so. as to complete the connection timed circuit.

of one tuned circuit of said subsequent between the inductance Where further increase of the overall selectivity of the amplifier, when not using the crystal, is desired it is possible so to arrange matters that the two tuned circuits of the crystal coupled stage are not arranged in parallel relation but function as a single tuned circuit. This may be.

effected by providing means for varying the efiective value of the condenser C to a value which differs from the value of the neutralizing condenser C3 by an amount necessary to produce any required degree of coupling.

Where it is desired to obtain wide. pass-band conditions, the change-over switch SE2 maybe arranged in the subsequent positions (e. g., t and 5). or in some of them, to connect damping resi'stances R4 and R5, of difiering values for each position, across the two tuned circuits.

What we claim is:

1. A band-pass filter comprising two tuned circuits, crystal resonator means coupling said tuned circuits, means for-neutralising the capacity of said crystal resonator, a pair of contacts each connected to one terminal of the crystal resonator means, a first switch means having a reactance connected therebetween and;a point common to both tuned circuits selectively operable to,contact one or the other of said pair 01 contacts to control the effective impedance of said tuned circuits in such manner that in one position oi said switch means said tuned circuits are tuned to the series-resonant frequency of said crystal and in another position of said switch means said- 2. A crystal filter of variable band-pass width comprising a. pair of tuned circuits, a crystal element for coupling said circuits having one terminal connected to one-circuit and a second terminal connected to the other circuit, a reactance element and a resistor each having one terminal connected to a point common to both timed circuits, and a muIti-position switchnn successive positions thereof adapted to connect the reactance element across one of said tuned circuits.

connect said reactance across the other of said tuned circuits, connect said reactance across both of said tuned circuits and simultaneously shortacross the reactance in addition to the last-named connections.

' 3. A multi-stage amplifier comprising a. plurality of cascade-connected electron discharge tubes provided with coupling means between successive tubes, at least one of said coupling means being variable, and at least another of said coupling means comprising a piezo-electric crystal, switch means for selectively short-circuiting the crystal or connecting the crystal to couple successive tubes, and means under the control of said switch means for varying the value of said variable coupling means when the crystal is short-circuited and for maintaining constant the value 01 said coupling means when the crystal is efiective.

4. A multi-stage amplifier comprising a plurality of cascade-connected electron discharge tubes each provided with an input and an output circuit, means for variably coupling an output and an input circuit of certain successive tubes, a piezo-electric crystal constituting the coupling means between an output and an input circuit of other successive tubes, switch means for selectively short-circuiting said crystal or connecting the crystal in coupling relation, and means under the control of said switch means for varying the value of said variable coupling means when the crystal is short-circuited and for maintaining constant the value of said coupling means when the crystal is efiective.

5. A multi-stage intermediate frequency amplifier comprising aplurality of cascade-connected electron discharge tubes each provided with a tuned input and a tuned output circuit, a piezo-electriccrystal coupling successive tuned output and tuned input circuits,- means for variably coupling the next successive tuned output and tuned input circuits, a reactance element and a resistor each {having one terminal connected to a point common to both tuned circuits having the crystal coupling'therebetween, and a multi-position switch .in successive positions thereof adapted to connect the reactance element across one of said tuned circuits, connect said reactance across the other of said tuned circuits, connect/said reactance across both of said tuned circuits and simultaneously shortcircuiting the crystal, 'and to shunt said resistor circuiting the crystal, and to shunt saidresistor across the reactance in addition to the lastnamed connections, a second multi-position switch controlled in unison with the first switch and associated with the variable coupling means so constructed and arranged that in the first three positions the degree ot coupling is maintained constant and in the next position the coupling is varied.

REGINALD BASIL ARMSTRONG. GEQRGE LAMBERT GRISDALE. ERNEST FREDERICK GOQDENOUGH. 

